1. Field of the Invention
This invention relates to a radiation imaging apparatus for measuring a two-dimensional distribution of radioactive isotope in a sample, and more particularly to a radiation imaging apparatus capable of imaging the area of an observing field many times as large as the effective area of the input surface of an image intensifier.
2. Description of the Prior Art
A photograph technique using a photographic film to record the distribution of radiation-emitters has been conventionally utilized in an autoradiographic technique for measuring the two-dimensional distribution of radioactive isotope in a sample, or in a radiographic technique for measuring transmissivity of a sample with respect to the radiation.
The photographic technique has many disadvantages including that a photographic film must be exposed to the radiation for several weeks to several months; that a monitoring operation can not be conducted while the photographic film is being exposed to the radiation, and that it is complicated and difficult to accurately measure the quantity of the detected radiation based on a blackening density of the light-exposed portion of the film.
Applicants have previously invented another technique (apparatus) as shown in FIG. 1, for measuring the distribution of the radiation-emitters, in which the radiation is converted into scintillation light by a scintillator (22) and then is converged on a recording apparatus comprising an image intensifier (25), an image pickup device (26) and an image memory (32) in order to record the distribution of the radiation-emitters. This apparatus is disclosed in detail in Japanese published unexamined application No. 296290/86 (published on Dec. 27, 1986). As shown in FIG. 1, the radiation emitted from a sample (23) situated on a sample-rest (21) is incident to a scintillator (22) which covers the sample, and is converted into scintillation light by the scintillator. The emitted scintillation light is converged through an optical lens (24) onto a photocathode of the image intensifier (25) in order to convert the scintillation light image into an amplified electron image. The electron image is amplified by a microchannel plate (MCP) in the image intensifier. The amplified electron image is converted into a phosphor image on the phosphor screen of the image intensifier. The phosphor image is projected through an optical lens (60) onto the image pickup device (26) followed by subsequent image processing.
The apparatus as shown in FIG. 1 overcomes the disadvantages associated with the photographic technique, and has an additional advantage that the magnitude of the area of the observing field can be optionally varied to some extent using an optical lens. However, the technique of FIG. 1 has the disadvantage that the light collecting efficiency of the technique is considerably low, i.e., ordinarily only about several percentages. For example, approximately 1% collecting efficiency is obtained if the diameter of the optical lens is 4 cm and the distance between the lens and an object to be measured is 10 cm. It is apparent from the above discussion that the technique of FIG. 1 can detect only a few photons in an emission phenomenon of ultra-low-level light, such as radioluminescence, and therefore is severely restricted in detection ability.
There has been proposed a method eliminating the use of an optical lens in which a scintillator and a sample are closely contacted to a fiber plate input-type of image intensifier. This method improves the collecting efficiency of the radio-luminescence to some extent, but has the disadvantage that the magnitude of the area of the observing field is limited to the effective area of the input surface of the image intensifier.